1. Field of the Invention
The present invention relates to carbon fibers (including graphite fibers) having novel physical properties and to a process for producing the same. More specifically, the invention provides a technical information of producing carbon fibers representing a peculiar load-elongation behavior, in an industrially advantageous manner, by a process wherein in the course of producing acrylic fibers, the spun filament bundle before the heat stretching treatment is subjected to a special stretching treatment in a warm water bath under a specific condition so that the degree of filament separability of the spun filament bundle traveling through the heat stretching step will be maintained within a prescribed range, and the acrylic fibers so prepared are heat-treated for carbonization.
2. Description of the Prior Art
It is well known to obtain carbon fibers which are excellent as reinforcing materials, exothermic elements, refractory materials, etc. by heating an acrylonitrile fiber in an oxidizing atmosphere at temperatures between 200.degree. and 400.degree. C. so as to form a cyclized structure in the fiber, followed by heating the cyclized fiber in a non-oxidizing atmosphere at higher temperatures (normally above 800.degree. C.).
However, the so-called thermal stabilization step, which is the step of forming naphthyridine rings in the fiber structure by heating the fiber in an oxidizing atmosphere, is a very important step that influences the physical properties of the carbon fiber which is the final product. It has been believed heretofore that this step requires a heat-treating operation for a long time and this has been the cause of the low productivity of carbon fibers.
If a condition of high-temperature thermal stabilization or a sharp temperature elevation is employed in order to increase the productivity of carbon fibers, abrupt reactions such as intermolecular cross-linking and intramolecular cyclization reactions will occur at a temperature in the vicinity of the exothermic transition point of the fiber. Accompanying such reactions, local accumulation of heat takes place which causes an uneven reaction to produce a pitch-like or tar-like substance. Such a substance causes mutual fusion of filaments (macro-fusion) or exerts a remarkable adverse influence on the physical properties of the carbon fiber, for example a decrease in mechanical strength.
Therefore, various processes have been proposed to accelerate the cyclization reaction so that thermally stabilized fibers can be obtained in a short time. All of these processes, however, have not necessarily contributed to the improvement in economy and industrial productivity of carbon fibers, because such processes are those copolymerizing a special comonomer with the fiber-forming polymer, or employing a treatment with a special chemical, or employing a complicated thermal stabilization step. By these means, the disadvantage of mutual "macro-fusion" of filaments can be eliminated to some extent, but there will still remain the phenomenon of micro-fusion, that is to say a phenomenon of fusion of carbon filaments in a very small number, e.g. from two to several tens of filaments. If carbon filaments with micro-fusion are used for the production of a carbon fiber-resin "composite material" (which is produced from carbon fibers impregnated with a resin, with tension applied to the carbon fibers), a phenomenon which we call "slipping-out" is caused. This remarkably lowers the shaping processability of the material. Thus, it is the present situation that a composite material of high commercial value, i.e. a carbon product of high strength, has not yet been provided.